It is common for direct fuel injected engines to have two fuel pumps. One example of a two pump direct injection fuel system is described in U.S. Pat. No. 6,230,688. This patent describes a fuel system wherein one fuel pump (i.e., the lift pump) lifts fuel from a fuel tank and delivers the fuel to a second fuel pump (i.e., the injection pump) at a first pressure. The second fuel pump increases the fuel pressure to a second pressure so that fuel can be directly injected into a cylinder. Current or voltage supplied to the lift pump is controlled in response to a pump speed sensor or in response to a pressure sensor.
The above-mentioned method can also have several disadvantages. In particular, the method requires a sensor to monitor the lift pump outlet pressure or the lift pump speed. This arrangement adds cost to the system and is therefore less desirable than a system that does not require sensors. In addition, the sensors may reduce system reliability as the system may not function as well if a sensor degrades.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method that offers substantial improvements.